The amount of wireless services supported by modern mobile devices, such as MP3 players, cellular phones, smart phones, laptops, video gaming devices, tablets, etc., have increased significantly during the last decade. These wireless services include voice call, Global Positioning System (GPS) coupled with an interactive map for navigation, Internet browsing, video call, gaming, music downloading, etc., and require increasingly higher data rates so that new protocols or new versions of an existing protocol are frequently released. These services are generally not deployed on the same frequency band in all countries, and the antennas used for the wireless communication have to cover many and/or wide frequency bands to support a wide variety of services with optimum data rates. However, for design and cosmetic reasons, most of the antennas used for the wireless communication are embedded within the device, in a very limited space, which has a negative impact on the bandwidth, the number of frequency bands and the efficiency of the embedded antennas, thereby limiting the availability and/or performances of the wireless services.
To overcome these issues, several solutions have been proposed over the years to increase the number of frequency bands and the bandwidth of each band that are supported by an antenna system. Typical solutions rely on a broadband matching circuit or addition of a parasitic element in the antenna to widen its operation range. However, in general, there are theoretical limits regarding how much the bandwidth of an antenna can be widened while keeping good enough performances. Another solution to address the problems mentioned above is to use multiple antennas, each supporting a subset of the frequency bands. In such an antenna system, relatively simple matching circuits can be designed for each antenna to maximize the bandwidth of each element. This type of solution, in which multiple antennas are utilized, can address the bandwidth problem; however, the physical volume allocated to the antenna system becomes large and is divided among the individual antennas, and thus radiation efficiency of each element tends to deteriorate.
As explained above, the volume of data transmission is required to be larger with even faster speed as the wireless services increase and QOS is further demanded. This motivates to obtain communication channels with wider bandwidths and efficient use of fragmented spectrum. For this purpose, the “carrier aggregation” scheme has been devised, wherein two or more component carriers are aggregated to support wide bandwidths. According to Release 10 of LTE-Advanced, for example, the data throughput is expected to reach 1 Gbps. Carrier aggregation may achieve a 100 MHz bandwidth by combining different carriers. There are three carrier aggregation modes to date: intra-band contiguous allocation, intra-band non-contiguous allocation and inter-band allocation. The intra-band contiguous allocation contiguously aggregates components carriers, each having about a 1.4 MHz bandwidth up to about a 20 MHz bandwidth, in one band. The intra-band non-contiguous allocation non-contiguously aggregates component carriers in one band, thereby having gaps between some of the component carriers. Note, however, that this carrier aggregation is not supported by the Release 10 at present time. The inter-band allocation aggregates component carriers in different bands, resulting in a non-contiguous allocation with gaps. Thus, the carrier aggregation scheme is expected to allow for simultaneous transmit and receive, which poses new challenges in RF front-end circuit and antenna designs, modulations/demodulations and various other RF techniques.